hooks/add-offb: Add leading slash to HVC node paths

We need an absolute path to the serial@N device. It looks like some
kernels aren't reading the current path correctly, leading to no console
output.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>